Only in New York could an alleyway spark a $10 million real estate battle.

Great Jones Alley is a tiny, 20-foot wide, 137-foot long sliver of pavement between Bond and West 3rd streets.

On one side is a slick new Noho condo called 1 Great Jones Alley, where the penthouse just sold for $30 million.

On the other side is 684 Broadway, a boutique, 22-unit co-op building with floor-to-ceiling windows and spacious lofts, where a four-bedroom is selling for $6 million.

Both buildings own part of the alley.

The luxury developer considers it a jewel of a selling point — describing it in sleek marketing as a “secret” entrance for a “private paradise,” secured by a majestic gate that can hold the paparazzi at bay. Glossy renderings even show a luxury car idling on quaint cobblestones.

The other building says it’s just an asphalt alley.

The co-op board of 684 Broadway is suing 1 Great Jones Alley to protect its interests in the alley, according to a new $10 million lawsuit the building filed last week in Manhattan Supreme Court.

It says Madison Realty — whose apartments have been featured on Bravo’s “Million Dollar Listing” — has been promoting a fantasy, and that the condo developer has no right to change their shared slice of pavement.

The developer is “not authorized to remove and/or alter the gate that sits at the entrance to the alley … [and] do not have the right to advertise the alley as ‘private’ to potential buyers,” according to court papers.

The developer of 1 Great Jones Alley cannot even allow cars to sit and idle in the space — without the consent of 684 Broadway, the suit contends.

The co-op wants the condo’s misleading ads to end, and are asking a judge to order 1 Great Jones Alley to tell their residents no cars, or changes, will be permitted.

“The board believes that part of the alley and gate belongs to 684 Broadway, and felt that they had to file this lawsuit to protect their property rights,” said 684 Broadway in a statement via its lawyer, Robert Brown.